地震 第2輯
Online ISSN : 1883-9029
Print ISSN : 0037-1114
1771年八重山地震津波発生域における精密地形調査と津波発生のメカニズムに関する一考察
松本 剛木村 政昭
著者情報
ジャーナル フリー

1993 年 45 巻 4 号 p. 417-426

詳細
抄録

Historical record shows that a large-scale tsunami named “the Great Meiwa Tsunami” or “the Yaeyama Earthquake Tsunami” occurred on 24th April 1771 in the sea region of Miyako and Yaeyama Islands, westernmost part of Ryukyu District caused by a large earthquake and the tsunami inundated up to the inside of the islands. Swath bathymetric survey in this area was conducted by the use of SEABEAM and HS-10 systems on board the Research Vessels KAIYO and YOKOSUKA, in November 1990, April 1991, January and April 1992. Precise topographic contour map was completed in the area of 22°40′N-24°10′N, 122°50′E-126°20′E, including the westernmost part of the Ryukyu Trench and tsunami source area (around 24.0°N, 124.3°E). The survey area is classified into five different topographic domains which are arranged zonally. Those are:
Zone 1: north of 24°00′N, which is characterised by the distribution of well developed submarine canyons along the southern coasts of Yaeyama Islands.
Zone 2: 23°40′N-24°00′N, which is characterised by deep sea fans and its large-scale subsidence edged by steep scarp near 23°55′N-24°00′N, 124°10′E-124°20′E. The observed relative height of the depression is about 200-300m.
Zone 3: south of 23°40′N on the landward slope of the trench, which is characterised by quite complicated topography such as several escarpments and ridges and troughs of which trend is not clearly recognized.
Zone 4: trench axis area with depth about 6500-6600m, which is characterised by broad plain. Maximum width of the axial plain is about 40km.
Zone 5: seaward slope of the trench, which is characterised by horsts and grabens which are samely distributed in the sea area of the south of Okinawa Island. The strike direction of the horsts and grabens is NW-SE off Miyako Islands, and WNW-ESE off Yaeyama Islands.
The 1771 Meiwa Tsunami is considered to be originated by the large-scale subsidence of the deep sea fans which is composed of soft sediment derived from the submarine canyons just north of the fans, and the wave might be propagated along the canyons from the position of the tsunami source.Tention of N-S direction is suggested by the topographic features in the survey area, and the tention stress in this area is considered to be the origin of the large-scale subsidence which induced the tsunami.

著者関連情報
© 社団法人日本地震学会
前の記事 次の記事
feedback
Top